In the field of thermal ink jet printhead circuit design for providing the circuitry necessary to drive printhead heater resistors, early approaches used separate electrical interconnects for the individual heater resistors. These approaches obviously imposed a significant limitation on resistor and interconnect packing density achievable on a given area of the printhead substrate surface. In an effort to increase this packing density relative to these early approaches, various designs have been suggested for integrating electrical drive circuitry and thin film heater resistors on a thermal ink jet printhead. One such design and construction is disclosed in U.S. Pat. No. 4,532,530 issued to Hawkins wherein it is proposed to utilize polycrystalline silicon feed lines on an integrated circuit substrate to electrically connect into the thermal ink jet printhead heater resistors. This approach allows the drivers and logic circuits to be cofabricated in the same steps used to manufacture the printhead.
Another construction for integrating drive circuitry with a thermal ink jet printhead is disclosed in U.S. Pat. No. 4,695,853 issued to Hackleman et al and assigned to the present assignee. In this latter approach, an X-Y multiplexing circuit is connected on a common integrated circuit chip with vertically constructed heater resistors and multiplexing diodes to selectively switch the diodes and resistors from conduction to non-conduction during a multiplexing operation.
In both of the above types of construction and other known methods of thermal ink jet printhead construction and driver circuit integration, the driving circuitry is located on one area of the thin film printhead substrate, and the heater resistors are located on another area of the printhead substrate. These design approaches still impose a significant limitation on the achievable packing density of both heater resistors and associated driving circuitry on a given printhead device surface area.